diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
index 0737733..f46780e 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
@@ -162,14 +162,13 @@ public function setValue($values, $notify = TRUE) {
       parent::setValue($values, FALSE);
       // Support setting the field item with only one property, but make sure
       // values stay in sync if only property is passed.
-      if (isset($values['target_id']) && !isset($values['entity'])) {
+      // NULL is a valid value, so we use array_key_exists().
+      if (is_array($values) && array_key_exists('target_id', $values) && !isset($values['entity'])) {
         $this->onChange('target_id', FALSE);
       }
-      // NULL is a valid value, so we use array_key_exists().
       elseif (is_array($values) && !array_key_exists('target_id', $values) && isset($values['entity'])) {
         $this->onChange('entity', FALSE);
       }
-      // NULL is a valid value, so we use array_key_exists().
       elseif (is_array($values) && array_key_exists('target_id', $values) && isset($values['entity'])) {
         // If both properties are passed, verify the passed values match. The
         // only exception we allow is when we have a new entity: in this case
@@ -215,7 +214,7 @@ public function onChange($property_name, $notify = TRUE) {
       $target_id = $property->isTargetNew() ? NULL : $property->getTargetIdentifier();
       $this->writePropertyValue('target_id', $target_id);
     }
-    elseif ($property_name == 'target_id' && $this->target_id !== NULL) {
+    elseif ($property_name == 'target_id') {
       $this->writePropertyValue('entity', $this->target_id);
     }
     parent::onChange($property_name, $notify);
diff --git a/core/modules/field/src/Tests/EntityReference/EntityReferenceItemTest.php b/core/modules/field/src/Tests/EntityReference/EntityReferenceItemTest.php
index d7b7980..dbc9037 100644
--- a/core/modules/field/src/Tests/EntityReference/EntityReferenceItemTest.php
+++ b/core/modules/field/src/Tests/EntityReference/EntityReferenceItemTest.php
@@ -310,4 +310,41 @@ public function testSelectionHandlerSettings() {
     $this->assertTrue($field->getSetting('handler') == 'views');
   }
 
+  /**
+   * Tests validation constraint.
+   */
+  public function testValidation() {
+    // The term entity is unsaved here.
+    $term = Term::create(array(
+      'name' => $this->randomMachineName(),
+      'vid' => $this->term->bundle(),
+      'langcode' => LanguageInterface::LANGCODE_NOT_SPECIFIED,
+    ));
+    $entity = EntityTest::create([
+      'field_test_taxonomy_term' => [
+        'entity' => $term,
+        'target_id' => NULL,
+      ],
+    ]);
+    $errors = $entity->validate();
+    // Using target_id of NULL is valid with an unsaved entity.
+    $this->assertEqual(0, count($errors));
+    // Using target_id of NULL is not valid with a saved entity.
+    $term->save();
+    $entity = EntityTest::create([
+      'field_test_taxonomy_term' => [
+        'entity' => $term,
+        'target_id' => NULL,
+      ],
+    ]);
+    $errors = $entity->validate();
+    $this->assertEqual(1, count($errors));
+    $this->assertEqual($errors[0]->getMessage(), 'This value should not be null.');
+    $this->assertEqual($errors[0]->getPropertyPath(), 'field_test_taxonomy_term');
+    // This should rectify the issue, favoring the entity over the target_id.
+    $entity->save();
+    $errors = $entity->validate();
+    $this->assertEqual(0, count($errors));
+  }
+
 }
diff --git a/core/modules/system/src/Tests/Entity/EntityReferenceFieldTest.php b/core/modules/system/src/Tests/Entity/EntityReferenceFieldTest.php
index 01dd0c1..53de70a 100644
--- a/core/modules/system/src/Tests/Entity/EntityReferenceFieldTest.php
+++ b/core/modules/system/src/Tests/Entity/EntityReferenceFieldTest.php
@@ -238,7 +238,6 @@ function testAutocreateApi() {
     });
     $this->assertUserAutocreate($entity, function(EntityInterface $entity, UserInterface $user) {
       $entity->user_id[0]->get('entity')->setValue($user);
-      $entity->user_id[0]->get('target_id')->setValue(NULL);
     });
     $this->assertUserAutocreate($entity, function(EntityInterface $entity, UserInterface $user) {
       $entity->user_id->setValue(array('entity' => $user, 'target_id' => NULL));
@@ -273,7 +272,6 @@ function testAutocreateApi() {
     });
     $this->assertUserRoleAutocreate($entity, function(EntityInterface $entity, RoleInterface $role) {
       $entity->user_role[0]->get('entity')->setValue($role);
-      $entity->user_role[0]->get('target_id')->setValue(NULL);
     });
     $this->assertUserRoleAutocreate($entity, function(EntityInterface $entity, RoleInterface $role) {
       $entity->user_role->setValue(array('entity' => $role, 'target_id' => NULL));